1. Field of the Invention
This invention relates to a cross coil type indicating instrument, and more particularly to a cross coil type indicating instrument suitable as a vehicle instrument such as a speed meter, revolution indicator or fuel gauge.
2. Description of the Related Art
An indicating instrument to be mounted on a vehicle is extensively employed which is designed as follows: The indicating instrument has a pair of coils which form magnetic fields perpendicular to each other. Current is allowed to flow in the pair of coils according to an amount of measurement, to cause the latter to form magnetic fields. As a result, the magnet rotor is rotated in the direction of the composite magnetic field due to the magnetic fields formed by the two coils, to turn the pointer thereby to indicate the amount of measurement.
FIGS. 9 and 10 show a movement of a conventional indicating instrument of this type. A movement 1 includes a coil bobbin 2 consisting of upper and lower bobbins 2a and 2b. Two coils 3a and 3b are wound on the coil bobbin 2 in such a manner that they are perpendicular to each other. A disk-shaped magnet rotor 4 with N and S poles is provided inside the coil bobbin 2 on which the coils 3a and 3b are wound as was described above. The magnet rotor 4 is fixedly secured to a rotary shaft 5, namely, a pointer shaft which is axially extended through the inside of a shaft supporting portion 2c of the coil bobbin 2. The magnet rotor accommodating portion of the coil bobbin 2 has a bearing recess 6 in its bottom at the center. The bearing recess 6 and the lower half of the magnet rotor accommodating portion are filled with silicon 7 high in viscosity.
The lower end portion of the rotary shaft 5 is rotatably supported on the bottom of the bearing recess 6. The lower half of the magnet rotor 4 is held in the silicon 7 to brake the rotary shaft 5. When no current is supplied to the coils 3a and 3b, the magnet rotor 4 is rotatable freely. On the other hand, when currents are supplied to the coils 3a and 3b, the magnet rotor 4 is turned by a predetermined angle.
A dial 8 is mounted over the movement 1, and a pointer 9 is fixedly secured to the end portion of the rotary shaft 5 which is protruded through the dial 8.
In FIG. 10, reference numeral 10 designates a movement casing which accommodates the coil bobbin 2 and shields the magnetic field.
The conventional indicating instrument thus constructed will be described in more detail with reference to FIGS. 11(a) through 13. When a voltage V.sub.0 cos .theta. is applied to the coil 3a and a voltage V.sub.0 sin .theta. is applied to the coil 3b as shown, currents according to the respective voltages flow in the coils 3a and 3b, so that the coils 3a and 3b form magnetic fields .phi..sub.1 and .phi..sub.2 respectively. Ideally, the magnetic fields thus formed are perpendicular to each other, and form a composite magnetic field .phi. in the direction which is obtained by combining the directions of the magnetic fields .phi..sub.1 and .phi..sub.2.
The magnitudes of the magnetic fields .phi..sub.1 and .phi..sub.2 are proportional to the voltages V.sub.0 cos .theta. and V.sub.0 sin .theta., respectively. Hence, the direction of the composite magnetic field .phi. is at the angle .theta.. Therefore, by allowing the angle .theta. to correspond to a predetermined amount of measurement, the direction of the composite magnetic field .phi. corresponds to the amount of measurement. Thus, the magnet rotor 4, i.e. the rotary shaft 5 is turned according to the composite magnetic field, to turn the pointer 9 over the dial 8 to indicate the amount of measurement.
The rotary shaft 5 is turned while being braked by the silicon 7 in the coil bobbin 2. This prevents the rotary shaft 5 and accordingly the pointer 9 from being unnecessarily vibrated. When no currents flow in the coils 3a and 3b as in the case where the motor vehicle is not in operation, the rotary shaft 5 is released, and therefore means (not shown) for returning the pointer 9 to the zero point operates to return the pointer 9 to the predetermined zero position.
In the above-described conventional indicating instrument, as shown in FIG. 9, the shaft supporting portion 2c of the coil bobbin 2 is substantially cylindrical. Therefore, when the coils 3a and 3b are wound on the coil bobbin 2, the coils 3a and 3b are guided and curved by the outer surface of the shaft supporting portion 2c. Hence, the resultant coils 3a and 3b are poor in alignment. In this case, the coils form unwanted magnetic fields in addition to those which are perpendicular to each other. Because of the unwanted magnetic fields thus formed, the indication of the pointer 9 is lowered in linearity; that is, the pointer 9 cannot indicate a correct value.